Cyclopentadienyl complexes of group 6 having at least one cyclopentadienyl system which is substituted by at least one silyl group which bears at least one halogen substituent and a catalyst system comprising at least one of the cyclopentadienyl complexes, and also a process for preparing them, the use of the catalyst system for the polymerization or copolymerization of olefins and a process for preparing polyolefins by polymerization or copolymerization of olefins in the presence of the catalyst system.